To be able to place the invention in its proper context and also to be able to demonstrate the value of the invention, a relatively detailed description of the state of the art as regards the use of a distance protection as a fault locator, and of the problems which are associated with the current technique within this field, will first be given. In this connection it is extremely important to have access to a measure of the load current prior to the occurrence of a fault.
The basic criterion for tripping of a power transmission line, in which a distance protection based on the impedance principle is used, is based on a check whether EQU .vertline.Z.vertline.=.vertline.U.sub.A /I.sub.A .vertline.&lt;Z.sub.in( 1)
that is, whether the numerical value of the impedance determined with the aid of phase voltage U.sub.A and phase current I.sub.A is smaller than a preset value Z.sub.in. This check can suitably be performed with a common underimpedance relay with a setting Z.sub.in lower than the normal load impedance.
When the distance protection is to be used as a fault locator, however, a considerable extension of the basic concept is required to achieve the desired accuracy and speed in the evaluation when a fault has occurred on the power transmission line.
Most fault locators are based on measuring the reactance between a short-circuit and that end of the power transmission line where the fault locator is placed. However, the accuracy in the distance calculation is influenced by the fault resistance. The reason for this is that the current which flows through the fault resistance is somewhat offset in phase in relation to the phase position of the current measured at the end of the power transmission line. This means that the fault resistance is interpreted as an apparent impedance with one resistive and one reactive component. It is, among other things, this reactive component which gives rise to the inaccuracy or the fault in the distance calculation since it influences the measured reactance.
The principles of fault location and calculation of fault resistance in connection with the occurrence of a fault on a protected line distance are known from a plurality of publications, some of which will be described below. The basic material consists of measured values obtained with the aid of instrument transformers for voltage and current at a measuring station adjacent to the protected line. These measured values are applied to a model of the network in question, which model is built into the distance protection. The current technique comprises A-D conversion and filtering of the measured values which then, via different distance protection equations for the model, determine the distance to the fault and the magnitude of the fault resistance.
A fault locator is described in an article entitled "An accurate fault locator with compensation for apparent reactance in the fault resistance resulting from remote-end infeed" published in IEEE Transaction on PAS, Vol. PAS-104, No. 2, February 1985, pp 424-436. Besides taking into account the impedance Z.sub.1 of the power transmission line, this fault locator also takes into account the source impedances of the power transmission line to be able correctly to describe the network and the effect of feeding to the fault point of current from both directions. According to this method, sampled phase currents I.sub.R, I.sub.S and I.sub.T, measured at a measuring station A at one end of the line and designated I.sub.A below, are memorized to be able to determine the change in the phase currents at the measuring station which arises when a fault occurs, that is, the current change I.sub.FA equal to the present phase current I.sub.A after the occurrence of a fault less the phase current prior to the occurrence of the fault. The method of obtaining a measure of the current change I.sub.FA described above requires an extensive memory capacity and the method of calculation is relatively time-consuming.
Because the current I.sub.F which flows through the fault resistance has a current contribution also from a supply station at the other end of the power transmission line, I.sub.F will be different from I.sub.FA. The relationship between these can be determined with the aid of the distribution factor of the network. The equations which can be set up in this way allow a possibility of determining both the current I.sub.F through the fault, the fault resistance and the distance to the fault.
Obtaining a measure of the current I.sub.F through the fault with the methods described above requires, as mentioned above, a considerable memory capacity, and because the method of calculation is relatively extensive, this is not a method which can be used when heavy demands are placed on fast protective functions. The reason for this is, among other things, that currents both prior to and after the occurrence of a fault must undergo a time-consuming Fourier filtering to obtain the fundamental components of the currents, freed from harmonics and d.c. components.
Swedish patent application SE 9203071-7 describes a fault model of a line network, which also takes into account the zero-sequence impedance of the network in that also the sum current I.sub.N, also called ground current, that is, EQU I.sub.N =I.sub.R +I.sub.S +I.sub.T =3.multidot.I.sub.0 ( 2)
where I.sub.R, I.sub.S and I.sub.T are the respective phase currents and I.sub.0 is the zero-sequence current, will be included in the equations which can be set up to determine the fault parameters.
Although, in principle, having access to the parameters of the network and the phase currents I.sub.A and I.sub.F and I.sub.N, it is now possible to determine the distance to a fault and the fault resistance, one practical problems remains, however, namely, as rapidly as possible after the occurrence of a fault, obtaining a sufficiently correct value of the phase currents immediately before and after the fault has occurred such that the desired accuracy in determining the fault parameters can be obtained.
Other methods for amplitude determination of the measured currents are also available. One such method comprises finding out the peak value with the aid of two consecutive sampled values for each cycle. Such a method is described, inter alia, in "High-speed distance relaying using a digital computer, Part 1--System Description", IEEE Trans on Power Apparatus and Systems, Vol-91, No. 3, May/June 1972, pp 1235-1243 by G. B. Gilchrest, G. D. Rockefeller and E. A. Udren. The peak values which are obtained in this way under normal conditions, that is, before the occurrence of a possible saturation of the current transformers, are relevant measured values which correspond to the Fourier amplitudes.